sockets exposing TC_DEFER_ACCEPT to optimise tcp exchanges.

UPGRADING

@@ -421,6 +421,9 @@ PHP 8.1 UPGRADE NOTES
     MYSQLI_REFRESH_SLAVE, in line with an upstream change in MySQL. The old
     constant is still available for backwards-compatibility reasons, but may
     be deprecated/removed in the future.
+
+- Sockets:
+  . TCP_DEFER_ACCEPT socket option added where available.
 
 ========================================
 11. Changes to INI File Handling

ext/sockets/sockets.c

@@ -541,6 +541,9 @@ static PHP_MINIT_FUNCTION(sockets)
 	REGISTER_LONG_CONSTANT("SOMAXCONN",		SOMAXCONN,		CONST_CS | CONST_PERSISTENT);
 #ifdef TCP_NODELAY
 	REGISTER_LONG_CONSTANT("TCP_NODELAY",   TCP_NODELAY,    CONST_CS | CONST_PERSISTENT);
+#endif
+#ifdef TCP_DEFER_ACCEPT
+	REGISTER_LONG_CONSTANT("TCP_DEFER_ACCEPT",   TCP_DEFER_ACCEPT,    CONST_CS | CONST_PERSISTENT);
 #endif
 	REGISTER_LONG_CONSTANT("PHP_NORMAL_READ", PHP_NORMAL_READ, CONST_CS | CONST_PERSISTENT);
 	REGISTER_LONG_CONSTANT("PHP_BINARY_READ", PHP_BINARY_READ, CONST_CS | CONST_PERSISTENT);

ext/sockets/tests/socket_defer_acp.phpt

@@ -0,0 +1,30 @@
+--TEST--
+Test if socket_set_option() works, option:TCP_DEFER_ACCEPT
+--SKIPIF--
+<?php
+if (!extension_loaded('sockets')) {
+        die('SKIP sockets extension not available.');
+}
+if (strpos(PHP_OS, 'Linux') === false) {
+	die('SKIP on non Linux');
+}
+?>
+--FILE--
+<?php
+echo "*** Test with TCP_DEFER_ACCEPT with initial SYN/ACK 'timeout' value to rounded up one ***\n";
+$socket = socket_create(AF_INET, SOCK_STREAM, SOL_TCP);
+
+if (!$socket) {
+        die('Unable to create AF_INET socket [socket]');
+}
+$initial_val = 5;
+var_dump(socket_set_option( $socket, SOL_TCP, TCP_DEFER_ACCEPT, $initial_val));
+socket_listen($socket);
+$rounded_up_val = socket_get_option( $socket, SOL_TCP, TCP_DEFER_ACCEPT);
+socket_close($socket);
+var_dump($rounded_up_val > $initial_val); // Value rounded up by the kernel, might differ from kernel version/setting
+?>
+--EXPECTF--
+*** Test with TCP_DEFER_ACCEPT with initial SYN/ACK 'timeout' value to rounded up one ***
+bool(true)
+bool(true)